Adhesion promoter in conversion solutions

ABSTRACT

An aqueous solution for producing a conversion layer on metal surfaces, which has a pH in the range 1.5 to 6 and which contains the complex fluorides of Ti, Zr, Hf, Si and/or B, in an amount such that the concentration of Ti, Zr, Hf, Si and/or B is 20 to 500 mg/l, and 0.1 to 2 g/l of adhesion-promoting lacquer additives and wherein the composition of the aqueous solution is selected so that no crystalline zinc-containing phosphate layer is produced on the metallic surface. The lacquer additive preferably contains an element M selected from the group B, Al, Si, Ti and Zr, wherein at least one hydroxyl group and one or more organic groups R are preferably bonded to the element M, wherein the organic group or organic groups R are selected independently of each other from hydrocarbon groups with 3 to 16 carbon atoms or from —(CH 2 ) x —Y groups, wherein x is an integer from 1 to 10 and Y represents a hydroxyl group, a mercapto group, a primary or secondary amine group, a carboxyl group, an acrylic or methacrylic acid group or an oxirane group or each represents a fragment of a molecule which contains one or more of the groups mentioned or acid groups.

[0001] This invention lies within the field of corrosion protection of metal surfaces in which a corrosion protective conversion layer is produced. This conversion layer is used as the adherent primer for a subsequent lacquer finish.

[0002] There is extensive prior art relating to the deposition of corrosion protective layers on polished metal surfaces to increase the corrosion protection. In the following, a few examples of documents are given which have the chromium-free treatment of aluminum surfaces in particular as the object. This type of treatment is generally also suitable for zinc surfaces and optionally also for iron surfaces. The expression “conversion treatment” used here means that the components in the treatment solution react chemically with the metal surface, which produces a corrosion protection layer in which both components from the treatment solution and also metal atoms from the metal surface are incorporated.

[0003] The chromium-free conversion treatment of aluminum surfaces with fluorides of boron, silicon, titanium or zirconium combined with organic polymers to achieve permanent corrosion protection and to produce a primer for subsequent lacquering is known in principle:

[0004] U.S. Pat. No. 5,129,967 discloses treatment baths for a no-rinse treatment (termed “dried in place conversion coating” therein) of aluminum, containing:

[0005] (a) 10 to 16 g/l of polyacrylic acid or homopolymers thereof,

[0006] (b) 12 to 19 g/l of hexafluorozirconic acid,

[0007] (c) 0.17 to 0.3 g/l of hydrofluoric acid, and

[0008] (d) up to 0.6 g/l of hexafluorotitanic acid.

[0009] EP-B-8 942 discloses treatment solutions, preferably for aluminum cans, containing:

[0010] (a) 0.5 to 10 g/l of polyacrylic acid or an ester thereof and

[0011] (b) 0.2 to 8 g/l of at least one of the compounds H₂ZrF₆, H₂TiF₆ and H₂SiF₆, wherein the pH of the solution is less than 3.5,

[0012] and also an aqueous concentrate for topping up the treatment solution containing:

[0013] (a) 25 to 100 g/l of polyacrylic acid or an ester thereof,

[0014] (b) 25 to 100 g/l of at least one of the compounds H₂ZrF₆, H₂TiF₆ and H₂SiF₆, and

[0015] (c) a source of free fluoride ions which provides 17 to 120 g/l of free fluoride.

[0016] DE-C-24 33 704 describes treatment baths to increase the adhesion of lacquer and for the permanent corrosion protection of, inter alia, aluminum which may contain 0.1 to 5 g/l of polyacrylic acid or salts or esters thereof and also 0.1 to 3.5 g/l of ammonium fluorozirconate, calculated as ZrO₂. The pH of these baths may vary over a wide range. The best results are generally obtained when the pH is 6-8.

[0017] U.S. Pat. No. 4,992,116 describes treatment baths for the conversion treatment of aluminum with a pH between about 2.5 and 5, which contain at least three components:

[0018] (a) phosphate ions in the concentration range between 1.1×10⁻⁵ and 5.3×10⁻³ mol/l corresponding to 1 to 500 mg/l,

[0019] (b) 1.1×10⁻⁵ to 1.3×10⁻³ mol/l of a fluorinated acid of an element selected from the group Zr, Ti, Hf and Si (corresponding to 1.6 to 380 mg/l, depending on the element) and

[0020] (c) 0.26 to 20 g/l of a polyphenol compound obtainable by reacting poly(vinylphenol) with aldehydes and organic amines.

[0021] The molar ratio of fluorinated acid to phosphate has to be maintained between about 2.5:1 and about 1:10.

[0022] WO 92/07973 discloses a chromium-free treatment process for aluminum which uses, as essential components, in acidic aqueous solution, 0.01 to about 18 wt. % of H₂ZrF₆ and 0.01 to about 10 wt. % of a 3-(N—C₁₋₄-alkyl-N-2-hydroxyethyl-aminomethyl)-4-hydroxystyrene polymer. Optional components are 0.05 to 10 wt. % of dispersed SiO₂, 0.06 to 0.6 wt. % of a solution-promoter for the polymer and a surfactant.

[0023] It is known that lacquer bonding agents may be added which improve the adhesion between the lacquer coating and the primer. Lacquers differ from solutions for conversion treatment in that the lacquer layer adheres to the substrate by forces of sorption and does not react chemically with the substrate. In contrast, a conversion solution reacts chemically with the substrate so that atoms from the substrate are incorporated in the conversion layer.

[0024] The present invention is based on improving the adhesion between a conversion layer and a lacquer layer applied thereto.

[0025] The present invention provides an aqueous solution for producing a conversion layer on metal surfaces which has a pH of from 1.5 to 6 and which contains the complex fluorides of Ti, Zr, Hf, Si and/or B, in an amount such that the concentration of Ti, Zr, Hf, Si and/or B is 20 to 500 mg/l, and 0.1 to 2 g/l of adhesion-promoting lacquer additives and wherein the composition of the aqueous solution is selected so that no crystalline zinc-containing phosphate layer is produced on the metallic surface. The aqueous solution preferably has a pH of from 2.5 to 4.5.

[0026] The complex fluorides of the elements mentioned may be introduced to the aqueous solution in the form of the corresponding fluorinated acid or its alkali metal and/or ammonium salts. However, it is also possible to form the complex fluorides only in the aqueous solution itself by reacting hydrofluoric acid or fluorides with ions of the metals mentioned. For example, complex fluorides of titanium or zirconium are produced by reacting oxides or salts of these elements with hydrofluoric acid.

[0027] In addition to the complex fluorides, the aqueous solution may contain free fluoride, for example in the form of hydrofluoric acid or of alkali metal or ammonium fluorides. The concentration of free fluoride may be, for example, from 0.001 to 1 g/l. This addition of free fluoride increases, in particular in the case of hot-dip galvanised steel or of aluminum, the pickling effect of the aqueous solution and thus the rate of conversion layer formation.

[0028] The aqueous solution preferably contains the complex fluorides of Ti, Zr, Hf, Si and/or B in an amount such that the concentration of Ti, Zr, Hf, Si and/or B is 50 to 400 mg/l.

[0029] The adhesion-promoting lacquer additives are preferably selected from water-soluble compounds which contain at least one element M selected from the group B, Al, Si, Ti and Zr. The compounds may contain one or more, in particular two, atoms of the elements mentioned. In the case of compounds which contain two or more atoms of the elements mentioned, it is preferred that they contain different elements M in the molecule. Compounds which contain both Al and Ti or both Al and Zr may be mentioned as examples.

[0030] Compounds which are preferred as adhesion-promoting lacquer additives are those in which at least one hydroxyl group and one or more organic groups R are bonded to the element M, wherein the organic group or organic groups R are selected independently of each other from hydrocarbon groups with 3 to 16 carbon atoms or from —(CH₂)_(x)—Y groups, wherein x is an integer from 1 to 10 and Y represents a hydroxyl group, a mercapto group, a primary or secondary amine group, a carboxyl group, an acrylic or methacrylic acid group or an oxirane group or each represents a fragment of a molecule which contains one or more of the groups mentioned or acid groups.

[0031] The at least one hydroxyl group may already be bonded to the element M when the compound is introduced into the aqueous solution for producing a conversion layer. It is also possible, however, that the hydroxyl group bonded to element M is produced only by reaction of a precursor compound with water in the aqueous solution for producing a conversion layer. For example, one or more alcoholate groups may be bonded to the element M and these are replaced by OH groups by reaction with water.

[0032] Groups which are particularly suitable as organic groups R are those which are known in silane chemistry as reactive groups for linking silanes with components in dyes and lacquers. Examples of such groups have been specified previously. Several different organic groups R may also be bonded to the element M. The secondary amine groups mentioned may be, for example, part of a diamine or polyamine unit. For example, part of an ethylene diamine or an ethylene diamine which is methyl-substituted at the terminal nitrogen atom. If, for example, Y represents an ethylene diamine group, this means that both a secondary and a primary amine group are present in organic group R. If Y represents a carboxyl group, then in the general case this is connected to the (CH₂)_(x) chain via the carbon atom in the carboxyl group. In the special case of an acrylic or methacrylic acid group, this is bonded to the (CH₂)_(x) chain via the carboxyl group itself, that is it forms an ester-like structure. Here, the double bond in the acid group is in the terminal position and may therefore be copolymerised with lacquer constituents. An oxirane group may form, for example, part of a glycidoxypropyl group.

[0033] Thus, Y may be not only one of the previously mentioned groups as such but also a molecular fragment containing one or more atoms which contains one or more of the previously listed groups or acid groups. For example, Y may represent a molecular fragment in which both a hydroxyl group and also a carboxyl group are present. The glycidoxypropyl group mentioned by way of example above is a molecular fragment in which both a hydroxyl group and also an oxirane unit are present.

[0034] In addition to the hydroxyl group or its precursor fragments and the previously listed organic groups R, further groups or ligands may also be bonded to the element M which do not react chemically under reaction conditions like those which prevail during use according to the present invention. For example, these may be alkyl groups or carboxyl groups. In particular when the molecule in the adhesive-promoting lacquer additive contains more than one, that is at least two, optionally different atoms of the elements M, these are generally linked via ligands. These may be, for example, carboxyl groups. However, hydroxyl groups or alcoholate groups may also take on this type of bridging function.

[0035] In the context of the present invention, those adhesion-promoting lacquer additives are preferred which contain Al or a combination of Al with one of the other elements M mentioned, in particular with Zr, as the element M.

[0036] The aqueous solution also preferably contains 50 to 2000 mg/l, in particular 100 to 1000 mg/l, of organic polymers. The organic polymers may be selected, for example, from epoxide resins, amino resins, tannins, phenol/formaldehyde resins, polycarboxylic acids, polymeric alcohols and esterification products thereof with polycarboxylic acids, poly-4-vinylphenol compounds, amino group-containing homopolymer or copolymer compounds and polymers or copolymers of vinylpyrrolidone. The use of these types of polymers in the field of metal surface treatment is known. They are characterised in more detail, for example, in DE-A-100 10 758, to which reference is made for a more detailed choice of preferred polymers.

[0037] Depending on the substrate, the aqueous solution may also contain 0.001 to 2, preferably 0.005 to 0.5 g/l, of ions of each of one or more of the metals Mn, Ce, Li, V, W, Mo, Mg, Zn, Co and Ni. These additional metal ions may further improve the corrosion protection effect and lacquer adhesion. For environmental reasons, however, the use of Co and Ni should be avoided.

[0038] Furthermore, the aqueous solution may also contain 0.001 to 1.5, preferably 0.1 to 1 g/l, of phosphoric acid, phosphorous acid, phosphonic acid and/or anions of each of these and/or esters of each of these. Esters should be selected which are soluble or dispersible in water. These additives also improve the corrosion protection effect and lacquer adhesion. However, in accordance with the basic principles of the present invention, it should be noted that those combinations of additives which lead to the formation of a crystalline zinc-containing phosphate layer should not be selected. In order to achieve this, for example, the treatment solution should not simultaneously contain zinc and/or manganese in concentrations of more than 0.3 g/l and phosphoric acid or phosphate ions in concentrations of more than 3 g/l.

[0039] It is advantageous, however, if the aqueous solution also contains one or more components which are known from the industrial field of phosphatising as so-called phosphatising accelerators. During a phosphatising process, these types of accelerators have the main task of preventing the formation of bubbles of elemental hydrogen on the metal surface. This effect is also termed a depolarisation effect. As in the case of conventional phosphatising, this also has the result with the solution according to the present invention that the formation of the conversion layer takes place more rapidly and that this is produced more uniformly. Accordingly, it is preferred that the aqueous solution contains one or more phosphatising accelerators selected from

[0040] 0.05 to 2 g/l of m-nitrobenzenesulfonate ions,

[0041] 0.1 to 10 g/l of hydroxylamine in the free or bonded form,

[0042] 0.05 to 2 g/l of m-nitrobenzoate ions,

[0043] 0.05 to 2 g/l of p-nitrophenol,

[0044] 1 to 70 mg/l of hydrogen peroxide in the free or bonded form,

[0045] 0.05 to 10 g/l of organic N-oxides,

[0046] 0.1 to 3 g/l of nitroguanidine,

[0047] 0.1 to 500 mg/l of nitrite ions, and

[0048] 0.5 to 5 g/l of chlorate ions.

[0049] Since a particular objective of the present invention is to avoid the use of toxic chromium compounds, it is preferred that the aqueous solution does not contain any chromium. Although the addition of chromium compounds to the aqueous solution may have a positive effect on the corrosion protection in individual cases, adequate corrosion protection and lacquer adhesion may also be produced using the present process without the use of chromium compounds in the areas of application concerned.

[0050] In principle, the aqueous solution which contains the individual components in the application concentrations required to produce the conversion layer may be prepared by dissolving the individual constituents in water in the desired application concentration at the site of application. In practice, however, the procedure is usually such that a concentrate consisting of one or more containers is transported to the user who prepares the ready-to-use solution at the site of application from the concentrate or concentrates by diluting with water. Accordingly, the present invention includes an aqueous concentrate present in one or more, in particular two, containers which produces an aqueous solution with the previously described composition after diluting with water by a factor of between 50 and 500, in particular between 100 and 300, and if required adjusting the pH. Whether the concentrate is sold in one or two containers depends in particular on whether it contains an organic polymer and whether this polymer is stable in the presence of the other constituents in the concentrate. If there is no organic polymer present or if this is stable in the aqueous concentrate in the presence of the other constituents, the concentrate is preferably provided in one container.

[0051] Certain organic polymers, such as, polyacrylates, however, are not generally stable in the presence of the inorganic components in the concentrate, but flocculate or lead to the formation of a gel. In this case, it is preferred to divide the concentrate into at least two containers, wherein one container preferably contains the complex fluorides and the other container preferably contains the organic polymers. The adhesion-promoting lacquer additives may be added to either the container with the complex fluorides or the container with the organic polymers, wherein the latter is preferred. However, the adhesion-promoting lacquer additives may also be provided in a third container and added only to the ready-to-use dilute solution of the other components.

[0052] Another embodiment of the present invention is a process for corrosion protection treatment of metal surfaces, wherein the metal surfaces are contacted with an aqueous solution of the previously described type for a period of between 0.5 and 5 minutes to produce a conversion layer.

[0053] The metal surfaces are preferably selected from those which may be used in the architectural field, in the construction of machines, in the furniture and domestic appliance industry and in the construction of ships, aircraft and vehicles. These are in particular surfaces of steel, of steel which has been alloy galvanised in an electrolytic or hot-dip process, and also of zinc, aluminum and magnesium as well as alloys at least 50 atom. % of which consists of iron, zinc, aluminum or magnesium.

[0054] The aqueous solution used in the present process preferably has a temperature between ambient temperature (about 15 to 20° C.) and about 70° C. The temperature range 25 to 40° C. is preferred. The metal surface may be contacted with the aqueous solution by sprinkling with the aqueous solution or by immersion in the aqueous solution or alternatively a no-rinse process. Generally, the aqueous solution is allowed to take effect on the metal surface in spraying or immersion processes for about 0.5 to about 5 minutes. A period of 1 to 3 minutes is preferred for spray processes and a period of 2 to 5 minutes is preferred for immersion processes. In a no-rinse process, the solutions are dried onto the metal surfaces, without an intermediate rinsing process, about 3 to about 30 seconds after application.

[0055] Car bodies or domestic appliances in particular are frequently manufactured from different materials. For example, steel and/or steels galvanised in different ways are combined with each other or with parts made of aluminum and/or magnesium or alloys of each of these. A particular strength of the present process is that even in these cases an effective corrosion protection layer is produced on the different materials. Accordingly, a particular embodiment of the present invention is characterised in that vehicle bodies or domestic appliances are treated which have surfaces of at least two materials selected from zinc, aluminum, magnesium and alloys of these materials with each other or with other alloyed constituents.

[0056] Lacquering of the metal surface with an electrolytically depositable dip-lacquer or with a powder coating usually follows the step forming the conversion layer (=corrosion protection treatment). Electrolytic dip-lacquering, in particular cathodic dip-lacquering, is preferred for vehicle bodies. Modern lead-free or low-lead cathodically depositable electrodeposition lacquers are also suitable for this purpose, that is those dip-lacquers which contain less than 500 mg of lead per kg of dry substance in the lacquer suspension. Domestic appliances may also be electrolytically dip-lacquered. For cost reasons, however, powder coating is performed in this case. Furthermore, powder coating is frequently preferred in the architectural field and in the furniture industry. Accordingly, the present process is preferably characterised in that lacquering after corrosion protection treatment takes place using an electrodeposition lacquer or a powder coating.

[0057] The present process is generally part of the sequence of processes including cleansing, conversion protection treatment and lacquering. One or more rinsing steps using industrial water, town water or fully deionised water are provided in between these process steps. Spray or immersion processes may be used for the rinsing steps. The examples show a typical sequence of steps.

EXAMPLES

[0058] Aluminum sheets of AA 6016 (AC 120) grade were used for the tests:

[0059] The sample sheets were subjected to the procedure outlined below, wherein all the steps were performed as immersion processes:

[0060] 1. Cleansing with an alkaline cleanser from the applicant: Ridoline® 124 (2% strength) and Ridoline® 120 WX (0.1% strength), 55° C., 5 minutes,

[0061] 2. Rinsing with fully deionised water,

[0062] 3. Rinsing with fully deionised water,

[0063] 4. Conversion layer formation by treating with a test product in accordance with Table 1 (pH 2.5, 30° C., 2.5 minutes),

[0064] 5. Drying by blowing with compressed air,

[0065] 6. Applying a TGIC-free polyethylene powder coating.

[0066] The following lacquer adhesion tests were performed using the sheets pre-treated in this way; the results are given in Table 2:

[0067] 1. Boiling test: 2 hours in fully deionised water at 100° C.

[0068] 2. Cross-hatching according to DIN EN ISO 2409

[0069] 3. Cross-hatching with 5 mm indentation, based on DIN EN ISO 2509 and DIN EN ISO 1520

[0070] 4. Mandrel flex test according to DIN EN ISO 1519 TABLE 1 Test products Test product Composition Comparison  75 mg/l Ti as TiF₆ ²⁻ 125 mg/l of poly-(5-vinyl-2-hydroxybenzyl)- N-methylglucamine (Henkel KGaA) Example 1  75 mg/l Ti as TiF₆ ²⁻ 500 mg/l of Chartwell B 515.1^((a)) (in ethylene glycol) Example 2  75 mg/l Ti as TiF₆ ²⁻ 500 mg/l of Chartwell B 515.1 W^((a)) (in water) Example 3  75 mg/l Ti as TiF₆ ²⁻ 500 mg/l of Chartwell B 5l5.4^((b)) (in ethylene glycol)

[0071] TABLE 2 Test results Erichsen indentation (5 Mandrel flex mm) in cross- test according Cross-hatching hatching to DIN EN ISO according to according to 1520 DIN EN ISO DIN EN ISO 5 mm mandrel 2409 after 2409 after after boiling Example boiling test boiling test test Comparison Gt 0 Gt 2 OK Example 1 Gt 0 Gt 1 OK Example 2 Gt 0 Gt 0 OK Example 3 Gt 0 Gt 0 OK 

1. An aqueous solution for producing a conversion layer on metal surfaces, which has a pH in the range 1.5 to 6 and which contains the complex fluorides of Ti, Zr, Hf, Si and/or B, in an amount such that the concentration of Ti, Zr, Hf, Si and/or B is 20 to 500 mg/l, and 0.1 to 2 g/l of adhesion-promoting lacquer additives and wherein the composition of the aqueous solution is selected so that no crystalline zinc-containing phosphate layer is produced on the metallic surface.
 2. An aqueous solution as claimed in claim 1, wherein the adhesion-promoting lacquer additive is selected from water-soluble compounds which contain at least one element M selected from the group B, Al, Si, Ti and Zr.
 3. An aqueous solution as claimed in claim 2, wherein at least one hydroxyl group and one or more organic groups R are bonded to the element M, wherein the organic group or organic groups R are selected independently of each other from hydrocarbon groups with 3 to 16 carbon atoms and from —(CH₂)_(x)—Y groups, wherein x is an integer from 1 to 10 and Y represents a hydroxyl group, a mercapto group, a primary or secondary amine group, a carboxyl group, an acrylic or methacrylic acid group or an oxirane group or each represents a fragment of a molecule which contains one or more of the groups mentioned or acid groups.
 4. An aqueous solution as claimed in claims 1 to 3, wherein it also contains 50 to 2000 mg/l of organic polymers.
 5. An aqueous solution as claimed in claim 4, wherein the organic polymers are selected from epoxide resins, amino resins, tannins, phenol/formaldehyde resins, polycarboxylic acids, polymeric alcohols and esterification products thereof with polycarboxylic acids, poly-4-vinylphenol compounds, amino group-containing homopolymer or copolymer compounds and polymers or copolymers of vinylpyrrolidone.
 6. An aqueous concentrate in one or two containers which, after dilution with water by a factor of between 50 and 500 and if required adjustment of the pH, produces an aqueous solution as claimed in one or more of claims 1 to
 5. 7. A process for the corrosion protection treatment of metal surfaces, wherein the metal surfaces are contacted for a period of between 0.5 and 5 minutes with an aqueous solution as claimed in one or more of claims 1 to
 5. 8. A process as claimed in claim 7, wherein lacquering takes place after the corrosion protection treatment.
 9. A process as claimed in claim 8, wherein lacquering is performed with an electrodeposition lacquer or with a powder coating. 